This invention relates to loudspeakers and more particularly to planar-type loudspeakers adapted to operate over a significant frequency range while still providing a substantially uniform pressure response and efficiency comparable to that of conventional loudspeakers.
Speakers are frequently mounted in a door panel, wall, ceiling or the like of a mobile conveyance (for example, an automobile, boat, or trailer), a home, or various commercial environments where the available space for the speaker is relatively narrow and conventional speakers cannot fit into such space without some esthetically undesirable projection of the speaker into the listening area. A class of speakers referred to as planar-type or flat panel speakers have been developed for such applications. These speakers generally involve one or more voice coil drive mechanisms connected to drive one or more diaphragms. While for speakers operating over a relatively narrow frequency range, a single diaphragm driven by a single voice coil can be utilized, for speakers intended to operate over a wider frequency range, two or more diaphragms may be required, which may be driven from a common voice coil, but are frequently driven by separate voice coils.
While these flat panel loudspeakers are ideally suited for applications where there is little depth available for mounting the speaker, these speakers also suffer from a number of limitations which has severally restricted their use. A principle disadvantage is that the acoustic pressure response of these speakers is significantly less uniform than that for conventional speakers, especially at higher frequencies. In theory, the fact that each diaphragm has a large number, possibly even thousands, of natural vibration modes in the frequency range of interest should provide a relatively uniform frequency response due to the substantially random behavior of the diaphragm; however, this has not turned out to be the case in practice. Recent attempts to optimize these speakers to achieve a more uniform response, including controlling the shape of the diaphragms and the position at which a voice coil is attached to each diaphragm, have not resulted in significant improvement, the acoustic pressure response for these speakers still being substantially less uniform than for conventional speakers. Another problem with the planar-type loudspeakers is that they have a relatively low efficiency compared to conventional speakers. Finally, while in theory these speakers should be relatively inexpensive to manufacture, various techniques which have been employed in the prior art in an effort to enhance the pressure response uniformity of these speakers has increased their cost so that such speakers are now generally significantly more expensive than comparable conventional speakers.
A need therefore exists for a planar or flat panel speaker which is capable of providing a substantially uniform acoustic pressure response, at least comparable to that of comparable conventional speakers, over a reasonably wide frequency band, including higher frequencies, while having an operating efficiency which is at least comparable to that of conventional loudspeakers and a manufacturing cost which is no greater than that for a comparable conventional loudspeaker, and preferably cheaper.
In accordance with the above, this invention provides a planar-type loudspeaker for operation in a selected frequency band. The loudspeaker includes at least two separate, substantially flat, substantially co-planar diaphragms and a common drive mechanism for the diaphragms. The diaphragms are each of a material and are dimensioned so that each has only a limited number of natural modes of vibration in the selected frequency band and that corresponding natural modes for the diaphragms are close enough to achieve beneficial mechanical coupling and acoustic interference so as to provide a generally uniform acoustic pressure response over the selected frequency band. The diaphragms are preferably coupled only through the drive mechanism. It is also preferable that there be only two natural modes of vibration in the selected frequency band.
For preferred embodiments, there are two co-axial, substantially circular diaphragms, an inner diaphragm and an outer diaphragm, with the drive mechanism being connected to both diaphragms. For preferred embodiments, the inner diaphragm has first and second natural modes F1in and F2in respectively, the outer diaphragm has first and second natural modes F1out and F2out respectively, F1out/F1in≈0.4 to 2.1 and F2out/F2in≈0.5 to 2.0. For all embodiments, extra material may be applied to at least one of the diaphragms to alter at least one of its natural modes in a predetermined way. For the preferred embodiments discussed earlier, the extra material is a piece of material having good damping properties which is attached at least near the center of the inner diaphragm. Similarly, the loudspeaker may include an outer frame with at least one of the diaphragms being adjacent to the frame and connected to the frame through a compliant surround, it being the outer diaphragm which is attached to the frame through the compliant surround for preferred embodiments,.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.